1. Field of the Invention
The present invention relates to a laser processing apparatus for use in laser beam processing, where the term "laser beam processing" is used herein to signify manufacturing operations performed on a workpiece by the action of a laser beam. Specifically, the invention relates to an apparatus whereby a plurality of operations can be executed simultaneously by respectively focused beams which have been split from an original beam, and is especially applicable to use of an excimer laser beam.
2. Description of the Related Art
In the field of laser processing, use of an excimer laser (having an oscillation wavelength in the ultra-violet region) as a light source has the advantages of enabling more precise machining to be achieved than is possible with other types of laser processing. In particular, the cutting effect of an excimer laser beam is based an abrasion (i.e. non-thermal) action, since the effects of high photon energy are utilized rather than thermal effects such as are produced by other types of laser beam. That type of abrasive cutting action, together with the short wavelength of the laser light, allows minutely accurate processing to be performed.
Until now, other types of laser have been generally used, such as the carbon dioxide gas laser and YAG laser, however wider use of the excimer laser processing apparatus can be expected, due to the above advantages.
FIG. 1 shows an example of a prior art excimer laser processing apparatus. In FIG. 1, numeral 1 denotes an excimer laser oscillator, 8 denotes a workpiece, 19 denotes a workpiece carrier, and 15 denotes a laser beam condensing lens, which is a converging lens. The laser beam emitted by the laser oscillator is condensed by the converging lens 15 to be incident on the surface of the workpiece 8, with that surface being positioned near the focal position of the lens 15, to thereby cut a hole through the workpiece 8. When the cutting has proceeded to a sufficient extent, the workpiece carrier 19 is then moved to shift the workpiece 8, such that the condensed laser beam will then fall upon the next position at which a hole is to be cut, and the above operation is repeated.
Usually, the energy density of an excimer laser beam will not be sufficient to achieve cutting of the workpiece, so that in general it is necessary to condense the laser beam so as to increase the energy density, by using the converging lens 15.
FIG. 2 shows another example of a prior art excimer laser processing apparatus. In this case,numerals 2, 3 denote respective lenses which in combination function as non-converging optics for condensing the laser beam to a beam of smaller cross-sectional size, which is passed through an array of apertures formed in a mask plate 11, to fall on a workpiece 8. As in the example of FIG. 1 the workpiece 8 is mounted on a workpiece carrier 19, which will be assumed to be movable. The apertures in the mask plate 11 are formed beforehand at respective positions where holes are to be cut in the workpiece 8, and is fitted closely adjacent to the work surface of the workpiece 8. That is, the condensed laser beam from the lens 3 irradiates the entire front face of the mask plate 11, to thereby cut a plurality of holes in the workpiece 8 simultaneously.
However such an apparatus has the basic disadvantage that much of the laser energy will be wasted, since only the part of the laser beam which passes through the mask apertures is actually utilized. It has therefore been proposed to split the original laser beam into a plurality of beams, which are transmitted along respective optical fibers to the apertures in the mask plate. In that way, it should be possible to achieve greater efficiency of utilization of the laser beam. Such an apparatus is described for example in the IBM Technical Disclosure Bulletin, Vol. 28 No. 4, pp 1738 to 1739, September 1985. With the latter apparatus, which is illustrated in FIG. 3, a laser beam produced by a laser oscillator 1 is condensed by a converging lens 15, then enters a beam scrambler 14 to thereby equalize the energy density distribution of the condensed beam. The resultant beam is then directed into one end of a bundle of optical fibers 5, with the fibers being bundled closely together at that end. The other ends of the optical fibers are mutually separated as shown, being respectively fixedly attached within a 2-dimensional array of apertures which are formed spaced apart at equal intervals in a supporting plate 12. A mask 13 is superimposed on the plate 12, having apertures formed therein at arbitrary positions. These positions are selected from a standard array of positions respectively corresponding to the ends of the bundle of fibers 5, so that it is possible to select various different patterns of laser beams to be extracted from the mask 13, by using respectively different masks. In that way, it is proposed to simultaneously cut an array of holes in a workpiece, (specifically a polyamid substrate) or a plurality of workpieces, by using the apparatus of FIG. 3, while achieving high efficiency of utilizing the laser beam energy.
However such prior art types of apparatus have respective disadvantages. Considering the apparatus of FIG. 1, since it appears that the entire laser beam can be focused to a small spot on the workpiece surface, there is a high efficiency of utilization of the laser beam, so that it might be expected that the speed of cutting each hole in the workpiece would be high, enabling a high speed of processing to be achieved. However in actual practice, and in particular when an excimer laser beam is used, such a high efficiency of operation cannot be achieved, for the following reasons. If the cutting action of an excimer laser beam is used, with the beam focused to converge in the workpiece as shown, it is found that material flies off from the workpiece, due to the abrasive effects of the high photon energy of the beam. The resultant dispersed material will coat the lens, thereby obstructing transmission of the laser beam, so that a hole cannot be cut to more than a certain depth. In addition, such material may damage the lens.
There are thus problems in increasing the speed of cutting by a laser beam, through increasing the energy density of the laser beam, by condensing the beam. In general, the cross-sectional size of an excimer laser beam is typically 20 mm by 10 mm or more, and it is only possible to use a part of the entire beam width. For example if 1 mm diameter holes are to be cut in polyamid material, then in general, saturation of the cutting speed (for each irradiation by the laser beam) will occur when the energy density of the laser beam reaches approximately 1 J/cm. In general, the energy density of an excimer laser beam is approximately 0.1 J/cm, so that even if the beam energy density is increased by only a factor of 10 times, that saturation level will be reached. Such an increase in beam energy density is achieved by a beam condensation factor of 4 times (i.e. a reduction of 4 times in the horizontal and vertical dimensions respectively, of the beam cross-sectional area). That is to say, assuming the portion of the laser beam cross-section which is actually utilized is circular in shape, and is 0.4 mm in diameter, then the necessary increase in energy density to reach the saturation level can be achieved by condensing that beam portion to a diameter of 0.1 mm. In the case of using an excimer laser beam having a cross-sectional size of 20 mm by 10 mm in that way, less than 0.1% of the overall beam energy density would be utilized, so that the utilization efficiency would be very low.
The above problem is true not only of excimer laser types of apparatus, but also applies to various other types of laser processing apparatus when these are used to cut a workpiece material which has a high absorption factor.
Such laser processing is generally performed by generating the laser beam as repetitive pulses, and it is possible to precisely control the degree of each processing operation (e.g. the depth to which a hole is cut) by supplying an appropriate number of laser beam pulses. The cutting speed can be increased by increasing the pulse repetition rate. However with a practical excimer laser, that repetition rate is limited to approximately several hundred Hz, and it is difficult to achieve any increase in the cutting speed by increasing that rate.
The apparatus of FIG. 2 represents an attempt to achieve a higher speed of processing, by cutting a pattern of a plurality of holes in the workpiece 8 simultaneously, through the use of the mask plate 11 having a suitable pattern of apertures for admitting portions of the condensed laser beam from the lens 3. However as described above, the efficiency of utilization of the laser beam is very low. As a result, problems would arise in practical application of such an apparatus to manufacturing processing, i.e. problems of high operating cost and low speed of cutting the holes in the workpiece (since the energy density of the laser light emitted through each mask aperture will be low).
It might be thought that it would be possible to achieve a higher speed of laser beam processing by applying the prior art apparatus of FIG. 3. However basic problems would arise, particularly when an excimer laser beam is used. Part of the problem is the aforementioned dispersion of material that is driven out of the workpiece, by the photon energy of the laser beam. With the apparatus of FIG. 3, the laser beam is first condensed by the converging lens 15, then directed into the beam scrambler 14, to pass out through respective ones of the set of optical fibers 5. However in that case a basic advantage of the original laser beam, i.e. a very small angle of divergence of the beam, is lost. Specifically, each of the resultant laser beams emitted from the optical fibers 5 (these being referred to in the following as the split laser beams) diverges with an angle of divergence that corresponds to the numerical aperture of the optical fiber (assuming that the angle of divergence of the incident beam on the fibers is zero). Usually, these optical fibers 5 would be formed of quartz fibers, in which case the numerical aperture of each fiber is typically approximately 0.2. Such an optical fiber is used for transferring ultra-violet radiation, and efficient transfer can be achieved even if the angle of divergence of the incident laser beam applied to the optical fiber is large. However in principle, the angle of divergence of the beam emitted from such an optical fiber cannot be made smaller than the angle of divergence of the incident beam that is applied to the input end of the fiber. If each optical fiber is perfectly straight, then the incident and emitted beam of each fiber will have the same angle of divergence. If an optical fiber is curved, then the angle of divergence of the emitted beam from the fiber will be larger than that of the incident beam.
For the same reasons, the beam emitted from the beam scrambler 14 in the apparatus of FIG. 3 cannot be made to have a small angle of divergence. Specifically, the beam scrambler 14 typically consists of a single quartz fiber, which is curved as required by the respective positions of the workpiece and the laser beam source. The angle of divergence of the emitted light from a quartz fiber is expressed by the following equation (1), which has been established by the assignees of the present invention as a result of measurements obtained from experiments: EQU .THETA..sub.o =(8D n.sup.2 /r+.THETA..sub.i.sup.2).sup.1/2 ( 1)
In the above, n denotes the refractive index of the quartz fiber, r denotes the minimum radius of curvature of the quartz fiber, .THETA..sub.i denotes the angle of incidence on the quartz fiber (full angle), D denotes the diameter of the quartz fiber, and .THETA..sub.o denotes the angle of emergence (full angle).
In general, an optical element such as the beam scrambler 14, formed of a single cylindrical quartz fiber, which as described above will be curved to some extent. In the case of a quartz fiber, the value of refractive index n will be 1.468. Also in general, the minimum radius of curvature of the beam scrambler 14 will be less than 500 mm, so that value of r will be assumed in the following. If the laser beam is assumed to have a cross-sectional size of 20 mm by 10 mm, then the diameter of the lens 15 should be 22.5 mm. Assuming the focal length of the lens 15 to be 225 mm, the angle of incidence .THETA..sub.i (full angle) of the emitted beam from the lens will be 0.1 radians. The bundle of optical fibers 5 will be assumed to consist of 100 quartz fibers, each having a cladding layer surrounding a core, with light being transmitted only through the core. The outer diameter of each fiber is 0.33 mm and the core diameter is 0.3 mm. If these are closely bundled together at the incidence ends of the fibers, to form a tubular shape, then the diameter of that tubular bundle will be 3.3 mm. Hence, the diameter D of the beam scrambler 14 should be 3.3 mm.
Inserting the above values for the beam scrambler 14 into equation (1) above, it is found that the angle of divergence (full angle) .theta..sub.o, i.e. the maximum of the angles of divergence of the emitted beam from the beam scrambler 14, will be 0.35 radians. Thus even if each of the optical fibers 5 were to be ideally formed with zero curvature, in order to minimize as far as possible the angle of divergence of the respective beams emitted from the fibers 5, the angle of divergence of these beams will be at least 0.35 radians. It will be apparent that the above would also be true if the beam scrambler 14 were to be made perfectly straight, but some or all of the quartz fibers 5 had a substantial degree of curvature (as would generally be necessary, in a practical apparatus).
It might be thought that it would be possible to reduce the angle of divergence at emission from the optical fibers by reducing the angle of divergence of the laser beam when emitted from the lens 15, so as to reduce the angle of incidence of the beam on the beam scrambler 14. However, if the diameter of the lens 15 is assumed to be fixed, a reduction in the angle of divergence of emission from that lens can only be achieved by increasing the focal length of the lens. The incidence end of the beam scrambler 14 would be set at the focal position of the lens 15, so as to minimize the size of the focused image of the laser beam source which falls on that end of the beam scrambler 14. However in fact the size of that focused image will be increased as a result of the original degree of divergence of the laser beam (when emitted from the laser oscillator), and that effect will be worsened as the focal length of lens 15 is increased. Hence, some of the laser beam will fall outside the incidence end of the beam scrambler 14, so that the efficiency of utilizing the laser beam will be reduced.
It can thus be understood that it is inherently impossible to obtain a small value of angle of divergence for the laser beams which are emitted from the optical fibers 5 of such an apparatus, i.e. in which the original laser beam is condensed by means of a converging lens before reaching the beam scrambler 14.
Moreover, if such an apparatus were to be used for laser beam processing, it would be necessary to set the respective emission ends of the optical fibers 5 in very close proximity to the surface of the workpiece, in order to ensure that a sufficiently high degree of energy density is maintained for the processing (e.g. for cutting of holes in the workpiece). However in that case it becomes impossible to prevent the emission ends of the optical fibers 5 from becoming coated by material which flies out from the workpiece as a result of the laser beam energy, so that emission of each laser beam from the fibers 5 rapidly becomes obstructed soon after emission has commenced, i.e. the optical transmission efficiency rapidly becomes very low, so that completion of cutting each hole by the laser beam cannot be achieved.
Furthermore if an excimer laser beam is utilized for such hole-cutting operations, then it is necessary for the energy density of a laser beam, when incident on the workpiece, to be higher than a certain value. However that value of energy density exceeds the light energy density withstanding capabilities of the types of optical fiber that are suitable for transferring an excimer laser beam. Hence it is inherently impractical to use the apparatus of FIG. 3, to perform cutting of holes in polyamid workpieces when using an excimer laser as the light source.
It might be considered that the above problem could be overcome, if the number of optical fibers is not prohibitively large, by providing respective converging lenses in correspondence with each of the apertures in the mask 13, for focussing the respective laser beams that are emitted through the mask. In that way, the emission ends of the optical fibers 5 could be disposed at a sufficient distance from the workpiece surface to prevent contamination by the dispersed material that is driven out of the workpiece by the laser cutting action. In addition, use of such lenses would enable the energy density of each of the laser beams extracted from the mask 13 to be increased to the requisite level by condensing the beam cross-sectional size, while the energy density of the laser beams transferred via the optical fibers 5 could be set to a sufficiently low value that is within the limitations of the optical fiber characteristics.
To propose a specific example, for the case of cutting holes in a workpiece formed of polyamid (which is suited to such laser beam processing), the required energy density at the workpiece is approximately 500 mJ/Pcm. If the optical fibers 5 are assumed to be quartz fibers, with the light source being an XeCl laser beam, then in order to ensure that the transmittance of the optical fibers will not deteriorate under long-term use, it is necessary to limit the laser beam energy density within the optical fibers 5 to approximately 200 mJ/Pcm. Thus, taking into consideration transmission losses within the optical fibers, etc., it is necessary to increase the energy density of each beam emitted from the optical fibers 5 by approximately 4 times, i.e. to condense the cross-sectional size of each emitted beam by the factor 1/2 in the horizontal and vertical directions. If it is assumed that each of these lenses is to be spaced apart from the workpiece surface by 300 mm (as a sufficient distance to prevent the lens from becoming obstructed by material driven out from the workpiece by the laser beam action), and assuming that the distance between each optical fiber emission end and the corresponding lens is to be made twice that value, i.e. 600 mm, so that the distance between the workpiece surface and each emission end of the optical fibers is 900 mm, and further assuming (based on the assumptions and calculation made hereinabove for the apparatus of FIG. 3) that the angle of divergence of each laser beam emitted from the optical fibers 5 is 0.35 radians, then in order to achieve a sufficient degree of light-gathering by such a lens it is necessary for the lens to have an effective diameter of approximately 200 mm (i.e. 600 mm.times.0.35). Moreover, since each of these lenses should have a focal length of 300 mm, the numerical aperture of the lens must be 0.33 (i.e. 105/(105+300). Such optically efficient lenses would be costly to manufacture, which is a serious problem. Furthermore since it is necessary of the effective diameter of each lens to be 200 mm, the apparatus would become large in size.
FIG. 4 shows an example of an apparatus in which a plurality of laser beams are transmitted to a workpiece surface, basically as described above using respective converging lenses for the optical fibers, and which is disclosed in Japanese Patent Application Laid-open Publication No. SHO 61-193794. In FIG. 4, a laser beam produced from a laser oscillator 31 is condensed by a converging lens 32 to enter a beam scrambler 33. The resultant laser beam from the scrambler 33, having uniform energy density, is supplied to the ends of a bundle of optical fibers 34, to be thereby split into a plurality of beams which are transmitted along respective fibers 35 of that bundle 34. Each of the fibers is coupled to a corresponding converging lens 38, with converging lens 38 being fixedly mounted in a control table 38. The control table 38 is movable horizontally and vertically (for focusing the split laser beams emitted from the lenses 38) under the control of a numerical controller 40, with respect to a workpiece 39 which is supported on a table 41. Each laser beam transmitted through the fibers 35 is condensed to a small-diameter region when incident on the workpiece 39, by the action of the corresponding one of the lenses 38. A plurality of processing operations can thereby be performed simultaneously by the action of the plurality of laser beams acting on the workpiece 39.
However as can be understood from the discussion hereinabove, there are various practical problems in implementing an apparatus such as that of FIG. 4, since each of the split laser beams emitted from the fibers 35 would have a large angle of divergence, it would necessary for the lenses 38 to be optically efficient (and therefore expensive), and for these lenses to be large in size.
Moreover, assuming that each of these lenses serves to condense the corresponding laser beam by a factor 1/2, then assuming for the reasons described hereinabove that the angle of divergence of each laser beam emitted from a fiber 35 is at least 0.35 radians, the angle of divergence at the workpiece surface will be twice that, i.e. 0.7 radians. Hence, the beam that is obtained from each of these lenses 38 will have a conical shape with a large angle of convergence, so that the range of workpiece thickness values in which the laser beam is highly concentrated will be small. Alternatively stated, if the workpiece is relatively thick, then the front diameter of a hole cut by such a laser beam will be substantially different from the diameter of the rear of the hole.
Another problem which must be overcome by an apparatus of the form shown in FIG. 4 is that of enabling the focus of each of the plurality of converging lenses 37 to be adjusted such that an identical, predetermined degree of condensing of the beams is achieved for each of the lenses 37, i.e. adjusted such that each of the laser beams correctly focused on the workpiece surface. In the example of FIG. 4, adjustment of focusing the beams transmitted from the fibers 35 onto the workpiece 39 is performed by vertically moving the control table 37, so that all of the heads and lenses 36, 38 are moved together. If the work surface of the workpiece 39 were absolutely flat, if the angles of divergence of the respective beams emitted from the optical fibers 35 were respectively identical, and the optical properties of the converging lenses 38 were mutually identical, with no manufacturing deviations, then such a method of focus adjustment would be possible. However in an actual manufacturing apparatus, such ideal relationships are not practical, so that if all of the emitted beams from the lenses 38 are adjusted vertically in common, uniform processing results (e.g. cutting an array of holes of respectively identical shape) will not be obtained by the actions of the respective beams.
For that reason, it would be preferable to provide means for individually adjusting the focus of each of the lenses which transmit the various laser beams to the workpiece, rather than to adjust all of these lenses in common.
One prior art method which has been proposed for such individual adjustment (in the case of an apparatus utilizing only a single laser beam) is described in Japanese Patent Application Laid-open Publication No. SHO 59-73192, and is illustrated in FIG. 5. That apparatus includes, in addition to a mechanism for such adjustment, a system for monitoring the respective intensities of light being transmitted towards, and reflected back from, the workpiece. In FIG. 5 a laser beam 52 emitted by a laser oscillator 51 is transferred through a beam-splitter 53, to be then reflected by a mirror 55 and pass through a converging lens 56. The lens 56 is fixedly retained in a movable lens mount 61, which can be adjusted in the vertical direction by rotating an adjusting screw 59. The focus of the lens 56 can thereby be adjusted so that the laser beam is condensed to a required degree, when incident on a workpiece 57 which is disposed on a table 60 below the lens mount 61. Part of the laser light reflected from the workpiece 57 is reflected back via the mirror 55 and beam splitter 53 to be measured by the power meter 58, while the level of the original laser beam 52 is measured by the power meter 54.
However in the case of a laser processing apparatus which utilizes a large number of optical fibers (for example, 100 fibers) to transmit respective laser beams to corresponding converging lenses, it would be necessary to provide a more simple arrangement for individually adjusting these converging lenses, in order to minimize the manufacturing cost of the apparatus.
Another problem of the prior art will be described referring to FIG. 6, which shows example of another type of laser processing apparatus. In FIG. 6, a laser beam emitted from a source 71 is transmitted through a converging lens 72, to fall on a set of incidence ends 74a of a large number of optical fibers 74, which are bundled together in close mutually contact at their incidence ends. The optical fibers are then curved outward as shown, with the other ends (i.e. the emission ends 74b) of the fibers being attached at approximately equidistant spacings around the periphery of a jig 75. The jig 75 is held positioned closely adjacent to a surface of a workpiece 76. Processing operations are thereby performed by respective laser beams emitted from the emission ends 74b of the optical fibers, with the outline of the result (e.g. set of welds or holes) thus produced being indicated as 76a.
With such an apparatus, in which a large number of small-diameter optical fibers are closely mutually bundled together at the incidence ends 74a, it is very difficult to prevent the optical fibers from mutually crossing over one another in the region close to the incidence ends 74a, i.e. a region in which the optical fibers are very closely bundled together. As a result of such intertwining of the fibers, the effective minimum radius of curvature of the fibers may be substantially reduced. As described hereinabove, the angle of divergence of the respective laser beams emitted from the optical fibers is strongly dependent on the minimum radius of curvature of each fiber. Thus, as a result of such cross-overs between the optical fibers at the incidence ends 74a, the energy density of the laser beams emitted from the optical fibers may not be sufficiently high optical fibers is disposed very close to the workpiece to perform the desired processing operation. That problem can similarly occur with the prior art apparatus of FIG. 4, within the optical fiber bundle 34. In that case, the angle of divergence of the laser beams emitted from some of the heads 36 may be excessively large, so that the energy density of the focused beams from the corresponding converging lenses 38 may not be sufficient. Reliable and uniform processing of the workpiece cannot be achieved in such a case.
As described hereinabove, a basic problem of laser beam cutting (particularly when using on the high photon energy of short-wavelength laser beam such as an excimer laser beam) is the effects produced by material driven out of the workpiece by the action of the laser beam. If the laser beam is condensed by a converging lens, before falling on the workpiece, so that the converging lens is positioned close to the workpiece surface, then the lens will rapidly become coated by the material driven out from the workpiece, or may be damaged by that material. One method which has been proposed in the prior art to overcome that problem is to mount each converging lens within a receptacle (generally of basically tubular shape, and referred to in the following as a processing head) provided with a nozzle. Each laser beam, after passing through the corresponding converging lens, emerges from the tip aperture of the nozzle to fall on the workpiece surface. A current of a gas is made to flow into each processing head, to be ejected from the nozzle aperture, and flow over the region of the workpiece that is being cut by the corresponding laser beam. Material driven out of the workpiece is thereby prevented from reaching the lenses.
Such a gas will be referred to in the following as "assist gas", and can have other functions besides protection of the lenses from obstruction or damage due to material driven out of the workpiece by the effects of laser beam cutting. Specifically, with some types of workpiece material it is essential that the laser beam processing operation (e.g. welding or cutting) take place in the presence of a specific gaseous atmosphere, typically formed of an inert gas such as helium. By passing a flow of such an assist gas through a nozzle as described above, such a condition can be established for a small region immediately surrounding each position of incidence of a laser beam on the workpiece.
However some types of assist gas, such as helium, are expensive. It is therefore necessary to minimize the consumption of the gas as far as possible, to achieve practicable processing costs. That can be done by making the size of each nozzle aperture as small as possible (to minimize the total rate of flow of the assist gas). However in that case it becomes necessary to accurately locate the axis of each laser beam in the center of the respective nozzle aperture. This is difficult to achieve in practice, due to manufacturing variations in the laser beam axis positions and in the positions of the nozzle apertures, and also because the laser beam axis may be periodically varying slightly in position. It has therefore been difficult to effectively achieve a desired reduction in the consumption of the assist gas.
One method which has been proposed in the prior art for overcoming that problem, in Japanese Patent Application Laid-open Publication No. HEI 2-108487, is illustrated in FIG. 7. In FIG. 7, gas from a pressurized cylinder 97 flows into an upper chamber containing a converging lens 98, which focuses a laser beam 95. A movable nozzle 91 is mounted at the lower end of the upper chamber, being laterally movable, for example under the control of a controller 92 which functions in accordance with a detected amount and rate of lateral movement of the laser beam 95 with respect to a workpiece 94 to move the nozzle 91 in the same direction and by the same amount of movement. It can be understood that such control of the laterally movable nozzle 91 enables the central axis of the laser beam 95 to be adjusted to be held continuously at the center of the tip aperture of the nozzle 91. The amount of allowable (uncorrected) mutual position deviation between the nozzle tip and the laser beam can thereby be doubled, by comparison with an apparatus in which the nozzle is fixed in position. Thus, the nozzle tip aperture can be made smaller, and the requisite rate of flow of the assist gas can thereby be reduced.
However it will be apparent that in the case of a laser processing apparatus producing a large number of laser beams which are transmitted through respective optical fibers and respective converging lenses, it would be impractical to provide such a complex nozzle-adjustment system for each of the laser beams, since the overall manufacturing cost of the apparatus would be greatly increased. There is therefore a requirement for achieving a similar result, but with a less complex arrangement, which would be applicable to a laser processing apparatus producing a large number of laser beams.
In a practical apparatus, when an original laser beam is split among a plurality of thin optical fibers to be transmitted through the fibers as in the example of FIG. 4, the respective laser beams which are emitted from the optical fibers will not have identical angles of divergence. Convergence lenses which are used to focus these laser beams on a workpiece are generally fixed within respective cylindrical mounts, i.e. processing heads, as mentioned hereinabove. Theoretically, the relationship between the lens diameter and the angle of divergence should be such that the diameter of each emitted beam, when reaching the corresponding converging lens, is the same as the lens diameter. However in practice it is found that variations in the angle of divergence occur between the various optical fibers which are coupled to the processing heads. Such variations are to be expected, since as described above referring to equation (1), the emission angle of divergence of a laser beam which is transmitted along an optical fiber will depend upon factors such as the diameter of the optical fiber and the minimum radius of curvature of the fiber, and these will be subject to manufacturing variations. Such variations in the angle of divergence result in variations in the convergence angles of the respective laser beams, when these emerge from the corresponding converging lenses to fall on the workpieces. Due to these variations in convergence angle between the laser beams of respective processing heads, it is difficult to achieve uniformity of processing by such an apparatus. For example, holes which are cut by beams from different processing heads may be respectively different in shape, as seen in cross-section.
Thus, such variations in the angle of divergence between respective optical fibers present an obstacle to achieving highly uniform processing of different workpieces simultaneously by such a multi-beam laser processing apparatus.
In an attempt to overcome the problem whereby the respective converging lenses for the multiple laser beams of such an apparatus tend to become damaged or obstructed by material which is driven out of the workpiece due to the effects of the laser beam (especially in the case of the excimer laser, due to the high photon energy of the beam), a proposal has been made in Japanese Patent Application Laid-open Publication No. SHO 61-137382 whereby these converging lenses are made replaceable. With that proposal, the lenses are mounted such as to facilitate such replacement. However these lenses contribute substantially to the overall cost of such a laser processing apparatus, and repeated replacement of the lenses would result in a significant increase in the operating expenses.
A proposal has also been made (in Japanese Patent Application Laid-open Publication No. SHO 63-154287) to utilize a flow of assist gas in such a way as to prevent such obstruction or damage to the converging lenses will occur. However such a method can only slow down the rate at which the lenses become coated or damaged by the material driven out of the workpieces, and over a long period of use of the apparatus the problem will gradually increase.
There is therefore a requirement for a laser processing apparatus which can overcome the various problems of the prior art set out above, so that rapid processing with low operating costs can be achieved.